This invention relates to providing for transfer of information among controller stations in a distributed control system. In particular it relates to a data highway, where the several stations or group of stations are arranged sequentially along one transmission line rather than being arranged radially around a central junction point or a master station. Electrical transmission lines have been used for effecting such information transfer, but they suffer from limited bandwidth, are subject to radio frequency interference, and can present a safety hazard. Fiber optic transmission lines do not suffer from the deficiencies cited for electrical lines, but their use in sequential multi-station applications has been limited by the cost and design constraint imposed by the difficulty of tapping fiber cables. The object of this invention is to effect a reliable fiber optic two-way communication system among many controller stations which may be distributed over a large area. The invention uses repeater stations in a bi-directional loop with a fiber optic data highway. It requires no signal mixing, switching, or splitting devices, and provides fault tolerance for either station failure or transmission line failure.
There are two major requirements of the communication system for an industrial process distributed control system. The first is that it be capable of serving numerous stations over distances commonly found in industrial process plants. The second is that it be fault tolerant. That is, the system must not fail as a result of a station fault or a transmission line fault.
The fault tolerance criterion could be met by employing a bi-directional in-line cabling arrangement with passive tapping at each station, but high insertion and tapping losses would preclude such a system serving more than a few stations, regardless of distance. A "star" configuration is also fault tolerant, and could serve a significantly larger number of stations, but practical system distances are limited. Furthermore, such systems have excessive cabling costs because each station is connected to the others through a central mixing, splitting device. In-line and ring repeater station systems have been described in the prior art, some using by-pass switches to guard against station failure, but such systems are still vulnerable to cable faults. In contrast with the prior systems, the present invention uses a bi-directional ring of repeater stations. Repeater action is halted at whichever station is transmitting to prevent perpetually circulating signals.